The distribution of gas in a liquid in the form of extremely small microbubbles, i.e., on the order of 100 micrometers (microns) or less, is important to a variety of applications. For example, the introduction of microbubbles at the boundary layer of a fluid moving through a pipe or over a ship's hull is used to reduce drag forces. Also, research on bubble coalescence in salt water is of value to the Navy because small microbubbles created in a ship's wake remain in suspension in seawater for a long time. These small microbubbles can be detected by a variety of means from above and below the water's surface thereby making a ship's track easy to detect and follow. Accordingly, the Navy is researching a variety of ways to reduce or eliminate the small microbubbles from a ship's wake. One approach is based on the coalescence of small microbubbles to form larger microbubbles that then quickly rise the water's surface where they can disperse into the air. Therefore, in such research, it is desirable to produce large quantities of small microbubbles in order to study bubble coalescence techniques.
Prior art methods of producing microbubbles include the injection or blowing of gas into a liquid through a plate of porous material (e.g., ceramic, mesh, wood, glass frit, polymer, etc.) or through a hollow needle. However, these simple approaches produce bubbles over a wide range of sizes (e.g., 250-1000 microns in diameter) and very few small microbubbles having diameters of 100 microns or less.
A more complex microbubble generator capable of producing bubbles ranging from 250-1000 microns in diameter is disclosed in U.S. Pat. No. 5,534,143. One embodiment of the invention includes a chamber packed with small inert particles through which a liquid effluent and oxygen or other gas are admitted under pressure to produce relatively large bubbles. A venturi chamber is positioned to then receive the large bubbles in the liquid effluent and oxygen to further reduce the size of the bubbles to between 250-1000 microns. However, the disclosed embodiment produces few bubbles that are 100 microns or less in size. Further, the set-up must be positioned close to where the bubbles are needed and may therefore not be suitable for use in pipe arrangements or near a ship's hull.
Another approach relies on the injection of a liquid into a liquid using a narrow jet at a gas/liquid interface in order to entrap gas into the flow of liquid therethrough. However, this approach is sensitive to set-up variations where even small variations cause a big variation in bubble quantity and their size range.
Still another approach involves the electrolysis of a liquid. While this method can produce large quantities of small microbubbles of 100 microns or less, it only works in certain liquids and solutions. In addition, the gas formed is a function of the electrolysis process and therefore cannot be selected. Still further, the noble metal wire required by the method can greatly add to the cost of a large scale bubble production apparatus.